1. Field of the Invention
The present invention generally relates to the field of integrated circuits. In particular, the present invention is directed to a clock dithering system and method during frequency scaling.
2. Background of the Invention
As micro-electronic devices become smaller and smaller, power consumption has become a critical concern for micro-electronic designers. In order to provide for low power consumption in microprocessors, designers have provided for dual frequency systems in which the clock of the system is set at a low frequency when the amount of information to be processed is small and set at a high frequency when the amount of information to be processed is large. The low frequency setting allows for lower power consumption during low usage states. However, changing the frequency back and forth between low frequency states and high frequency states creates a significant amount of power noise in the system due to the sudden changes in current requirements. These power fluctuations on the processor itself create noticeable performance problems.
Prior attempts to reduce this noise generation include turning off portions of the processor or stopping clocks while changing the frequency. However, this solution also halts any information handling during the frequency change. Another method of changing the frequency includes gradually changing the frequency input into a phase locked loop (PLL) circuit. Due to the characteristics of most PLL's, changing the frequency using this method takes a significant amount of time to complete and the range of frequency change is greatly limited by the boundaries of the PLL.
Clock dithering has been used to constantly modulate a frequency signal for the purpose of reducing electromagnetic interference (EMI) emitted from electronic devices. Such EMI may interfere with other electronic devices in the vicinity. For example, U.S. Pat. No. 6,404,260 to Cruz-Albrecht describes the use of a non-periodic signal to modulate the period of a clock signal in order to reduce the size of spikes of electromagnetic radiation generated by a circuit that uses the clock signal. In another example, clock frequency modulation is described as a method of reducing EMI (See “Frequency Modulation of System Clocks for EMI Reduction,” by Cornelis D. Hoekstra, Hewlett-Packard Journal, August 1997). However, the clock dithering in these applications has been a constant modulation of the clock frequency to reduce interference with other devices and does not address the in-system noise generated during the change of a clock from a first frequency to a second frequency.
Accordingly, there is a need for a system and a method of changing the frequency of an integrated circuit device from one frequency to another quickly while reducing on-system noise and having the ability to process information during the frequency change.